This invention relates to a structure for compensating motion on an offshore platform. More particularly, but not by way of limitation, this invention relates to a structure and method to compensate for motion of an offshore platform due to tidal, wave, wind and other environmental factors.
In the exploration, drilling and production of hydrocarbons, operators search in remote and exotic areas of the globe. Deep water tracts have been explored and drilled with increasing frequency in recent years. Platforms set in waters of 1,000 to 2,000 feet has become common place, and in some instances, wells have been drilled in water depths of 5,000 feet. Different types of drilling and production platforms have been used in these deep waters. One type of platform is a tension leg platform (TLP). In the TLP, a floating platform is connected to the ocean floor via tendons such as steel cables, as is well understood by those of ordinary skill in the art. Another type of structure used in deep water is the spar platform which generally is a floating cylindrical structure that is anchored to the ocean floor with steel cable means. Other types of floating platforms are known in the art. In deep water, a fixed leg type platform is generally not an option due to the extreme water depths.
In the deep water drilling of subterranean reservoirs, drillers encounter numerous operational problems. For instance, wave conditions may cause a cyclic buoyant force based on the raising, lowering, heaving and pitching of the platform. Also, tidal conditions may cause a variation in platform height and cause similar buoyant forces. The applied forces will in turn cause motion on the platform and on the work deck of the platform. Additionally, the subterranean well that is drilled will have a riser extending from the sea floor to the platform. In other words, a riser extends from the sea floor to the floating platform. As will be understood by those of ordinary skill in the art, the riser generally does not move in unison with the platform since the riser is fixed to the sea floor by different attachment means and the riser does not experience the same buoyant forces as the floating platform.
While an operator is in the midst of performing well work, the motion of the platform can have detrimental effects on the equipment and ongoing operations. For example, a coiled tubing unit that is rigged-up and running a string of tools into the well could be lifted upward and/or downward due to the motion of the platform. This motion could potentially cause serious damage such as breaking the connection of the coiled tubing to the riser which in turn could lead to a catastrophic failure. With prior art designs, operators find it necessary to stop operations and rig down the connection and then reconfigure. Thus, there is a need for a system and method that can compensate for motion of a floating platform while undergoing well intervention procedures. This need, and many other needs, will be fulfilled according to the teachings of the present invention.